1,421 research outputs found
Multi-Transition Study of M51's Molecular Gas Spiral Arms
Two selected regions in the molecular gas spiral arms in M51 were mapped with
the Owens Valley Radio Observatory (OVRO) mm-interferometer in the 12CO(2-1),
13CO(1-0), C18O(1-0), HCN(1-0) and HCO+(1-0) emission lines. The CO data have
been combined with the 12CO(1-0) data from Aalto et al. (1999) covering the
central 3.5kpc to study the physical properties of the molecular gas. All CO
data cubes were short spacing corrected using IRAM 30m (12CO(1-0): NRO 45m)
single dish data. A large velocity gradient (LVG) analysis finds that the giant
molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180pc
(120pc) resolution with an average kinetic temperature of T_kin = 20(16)K and
H_2 density of n(H_2) = 120(240)cm^(-3) when assuming virialized clouds (a
constant velocity gradient dv/dr. The associated conversion factor between H_2
mass and CO luminosity is close to the Galactic value for most regions
analyzed. Our findings suggest that the GMC population in the spiral arms of
M51 is similar to those of the Milky Way and therefore the strong star
formation occurring in the spiral arms has no strong impact on the molecular
gas in the spiral arms. Extinction inferred from the derived H_2 column density
is very high (A_V about 15 - 30 mag), about a factor of 5-10 higher than the
average value derived toward HII regions. Thus a significant fraction of the
ongoing star formation could be hidden inside the dust lanes of the spiral
arms. A comparison of MIPS 24um and H_alpha data, however, suggests that this
is not the case and most of the GMCs studied here are not (yet) forming stars.
We also present low (4.5") resolution OVRO maps of the HCN(1-0) and HCO+(1-0)
emission at the location of the brightest 12CO(1-0) peak.Comment: 41 pages, 12 figures, 7 tables; accepted for publication by Ap
A New High Resolution CO Map of the inner 2.'5 of M51 I. Streaming Motions and Spiral Structure
[Abridged] The Owens Valley mm-Array has been used to map the CO 1--0
emission in the inner 2'.5 of the grand design spiral galaxy M51 at 2''-3''
resolution. The molecular spiral arms are revealed with unprecedented clarity:
supermassive cloud complexes, Giant Molecular Associations, are for the first
time resolved both along and perpendicular to the arms. Major complexes occur
symmetrically opposite each other in the two major arms. Streaming motions can
be studied in detail along the major and minor axes of M51. The streaming
velocities are very large, 60-150 km/s. For the first time, sufficient
resolution to resolve the structure in the molecular streaming motions is
obtained. Our data support the presence of galactic shocks in the arms of M51.
In general, velocity gradients across arms are higher by a factor of 2-10 than
previously found. They vary in steepness along the spiral arms, becoming
particularly steep in between GMAs. The steep gradients cause conditions of
strong reverse shear in several regions in the arms, and thus the notion that
shear is generally reduced by streaming motions in spiral arms will have to be
modified. Of the three GMAs studied on the SW arm, only one shows reduced
shear. We find an expansion in the NE molecular arm at 25'' radius SE of the
center. This broadening occurs right after the end of the NE arm at the Inner
Lindblad Resonance. Bifurcations in the molecular spiral arm structure, at a
radius of 73'', may be evidence of a secondary compression of the gas caused by
the 4/1 ultraharmonic resonance. Inside the radius of the ILR, we detect narrow
(~ 5'') molecular spiral arms possibly related to the K-band arms found in the
same region. We find evidence of non-circular motions in the inner 20'' which
are consistent with gas on elliptical orbits in a bar.Comment: 29 pages, 15 figures, uses latex macros for ApJ; accepted for
publication in Ap
Spin Fluctuations and Unconventional Superconductivity in the Fe-based Oxypnictide Superconductor LaFeAsO_0.7 probed by 57Fe-NMR
We report Fe-NMR studies on the oxygen-deficient iron (Fe)-based
oxypnictide superconductor LaFeAsO ( 28 K) enriched by
Fe isotope. In the superconducting state, the spin component of
Fe-Knight shift decreases almost to zero at low temperatures
and the nuclear spin-lattice relaxation rate exhibits a
-like dependence without the coherence peak just below , which
give firm evidence of the unconventional superconducting state formed by
spin-singlet Cooper pairing. All these events below are consistently
argued in terms of the extended s-wave pairing with a sign reversal of
the order parameter among Fermi surfaces. In the normal state, we found the
remarkable decrease of upon cooling for both the Fe and As sites,
which originates from the decrease of low-energy spectral weight of spin
fluctuations over whole space upon cooling below room temperature.
Such behavior has never been observed for other strongly correlated
superconductors where an antiferromagnetic interaction plays a vital role in
mediating the Cooper pairing.Comment: 4 pages, 4 figures,Accepted for publication in J. Phys. Soc. Jpn.,
vol.78, No.1 (2009
Origin of Tc Enhancement Induced by Doping Yttrium and Hydrogen into LaFeAsO-based Superconductors: 57Fe, 75As, 139La, and 1H-NMR Studies
We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of
La_{0.8}Y_{0.2}FeAsO_{1-y} (La_{0.8}Y_{0.2}1111) and
LaFeAsO_{1-y}H_{x}(La1111H), where doping yttrium (Y) and hydrogen (H) into
optimally doped LaFeAsO_{1-y} (La1111(OPT)) increases T_c=28 K to 34 and 32 K,
respectively. In the superconducting (SC) state, the measurements of
nuclear-spin lattice-relaxation rate 1/T_1 have revealed in terms of a multiple
fully gapped s_\pm-wave model that the SC gap and T_c in La_{0.8}Y_{0.2}1111
become larger than those in La1111(OPT) without any change in doping level. In
La1111H, the SC gap and T_c also increase slightly even though a decrease in
carrier density and some disorders are significantly introduced. As a
consequence, we suggest that the optimization of both the structural parameters
and the carrier doping level to fill up the bands is crucial for increasing T_c
among these La1111-based compounds through the optimization of the Fermi
surface topology.Comment: 4 pages, 4 figures, 1 table, to be published in J. Phys. Soc. Jpn,
Vol. 79, No. 1
Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study
We report the results of Se--nuclear magnetic resonance (NMR) in
-FeSe, which exhibits a similar crystal structure to the
LaFeAsOF superconductor and shows superconductivity at 8 K. The
nuclear-spin lattice relaxation rate shows behavior below the
superconducting transition temperature without a coherence peak. The
const. behavior, indicative of the Fermi liquid state, can be seen in a
wide temperature range above . The superconductivity in -FeSe is
also an unconventional one as well as LaFeAsOF and related
materials. The FeAs layer is not essential for the occurrence of the
unconventional superconductivity.Comment: 4pages, 4figures, to be published in J. Phys. Soc. Jpn. 77 No.11
(2008
59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)
We report an NMR investigation of the superconductivity in BaFe(2)As(2)
induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with
Fe atoms and donate carriers without creating localized moments. Our finding
strongly suggests that the underlying physics of iron-pnictide superconductors
is quite different from the widely accepted physical picture of high Tc
cuprates as doped Mott insulators. We also show a crossover of electronic
properties into a low temperature pseudo-gap phase with a pseudo-gap Delta
560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc
decreases for both along the c-axis and ab-plane, and is consistent with the
singlet pairing scenario.Comment: Accepted for publication in J. Phys. Soc. Jpn. (4 pages
Low-lying excitations at the rare-earth site due to rattling motion in the filled skutterudite LaOs_4Sb_{12} revealed by ^{139}La NMR and ^{121/123}Sb NQR
We report experimental results of nuclear magnetic resonance (NMR) at the La
site and nuclear quadrupole resonance (NQR) at the Sb site in the filled
skutterudite LaOsSb. We found that the nuclear spin-lattice
relaxation rate divided by temperature at the La site exhibits a
different temperature dependence from that at the Sb site. Although at
the Sb site is explained by the Korringa mechanism, at the La site
exhibits a broad maximum around 50 K, showing the presence of an additional
contribution at the La site. The additional low-lying excitations observed at
the La site can be understood with the relaxation from anharmonic phonons due
to the rattling motion of the La atoms.Comment: 5 pages, 4 figures, final version published in Phys. Rev. B (Rapid
Communications
(Extra)Ordinary Gauge/Anomaly Mediation
We study anomaly mediation models with gauge mediation effects from
messengers which have a general renormalizable mass matrix with a
supersymmetry-breaking spurion. Our models lead to a rich structure of
supersymmetry breaking terms in the visible sector. We derive sum rules among
the soft scalar masses for each generation. Our sum rules for the first and
second generations are the same as those in general gauge mediation, but the
sum rule for the third generation is different because of the top Yukawa
coupling. We find the parameter space where the tachyonic slepton problem is
solved. We also explore the case in which gauge mediation causes the
anomalously small gaugino masses. Since anomaly mediation effects on the
gaugino masses exist, we can obtain viable mass spectrum of the visible sector
fields.Comment: 24 pages, 10 figure
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